The present invention relates to a semiconductor memory device and, particularly, to a technology that can be particularly effective when adapted to a memory system for multi-level data in a nonvolatile semiconductor memory device, such as a nonvolatile memory device (hereinafter simply referred to as a flash memory) which is capable of electrically erasing a plurality of stored data at one time.
In a flash memory, a nonvolatile memory element having a control gate and a floating gate is used as a memory cell, and the memory cell is constituted by a single transistor. In such a flash memory, the programming operation is carried out by applying a voltage of about 4 V(Volt) to the drain region of the nonvolatile memory element,as shown in FIG. 21, applying a voltage of about xe2x88x9210 V to a word line to which a control gate CG is connected, and discharging the electric charge from the floating gate FG by using a tunneling current to establish a state of a low threshold voltage (logic xe2x80x9c0xe2x80x9d). To carry out the erasing operation,as shown in FIG. 22, a voltage of about xe2x88x923 V is applied to the well region, to the drain region and to the source region, and a voltage of as high as 10 V is applied to the control gate CG to generate a tunneling current in order to inject a negative charge into the floating gate FG and to maintain a high threshold value (logic xe2x80x9c1xe2x80x9d). Thus, data of one bit is stored in a memory cell.
There has been proposed the concept of a so-called xe2x80x9cmulti-levelxe2x80x9d memory to store data of two or more bits in a single memory cell in order to increase the storage capacity. An example of such a multi-level memory has been disclosed in, for example, Japanese Patent Application H7-14031 (14031/1995) which corresponds to U.S. patent application Ser. No.08/860,793.
In such a multi-level memory of the above-mentioned patent application, consecutive two-bit data xe2x80x9c01xe2x80x9d, xe2x80x9c00xe2x80x9d, xe2x80x9c10xe2x80x9d and xe2x80x9c11xe2x80x9d are subjected to logical conversion, and are stored correspondedly to one of the threshold voltage ranges of 1.2V or lower, 1.6 to 2.3 V, 2.8 to 3.5 V, and 4 V or higher of the memory cell as shown in FIG. 23. In the above-mentioned memory, therefore, it is impossible to identify the stored data unless a reading operation is executed three times while successively changing the level of the word line to, e.g., 1.4 V, 2.6 V, 3.7 V, causing a drawback in that a long time is needed to read the data.
The object of the present invention is to provide a multi-level storage-type nonvolatile semiconductor memory device for which the reading time can be shortened by decreasing the number of accesses to the word line when reading the data, and a method of driving such a device.
The above and other objects, and novel features of the present invention will become obvious from the following description and the accompanying drawings.
The outline of a representative example of the invention disclosed in this application will be described below.
That is, in a nonvolatile semiconductor memory device in which a plurality of threshold values are set to store multi-level data in a memory cell, each bit of multi-bit data is separately written into a memory cell depending upon an address signal or a control signal and the bits are stored hierarchically. In this case, data of a plurality of bits may be consecutively written in one memory cell, of after data has been written in all memory cells bit by bit, the data of remaining bits may be successively overwritten in the memory cells.
Thus, when two bits are stored in one memory cell, the first bit can be read out by accessing the word line only one time and the succeeding bit can be read out by accessing the word lines only twice while changing the level to read the data. Thus, the total number of accesses to the word line is decreased, and the time taken to read data is shortened.